This proposal addresses the significant clinical and scientific challenge of targeted drug resistance in HER2- positive breast cancer. Breast cancer is the most common cancer in women in the US and HER2-positive disease is particularly aggressive. Trastuzumab, a monoclonal antibody targeted at the HER2 receptor, has revolutionized care for patients with HER2-positive disease, but resistance and subsequent disease progression occur frequently. While there have been multiple attempts to predict response to trastuzumab, previous studies often use bulk expression data from a single time point and do not consider the role of tumor evolution in treatment response. The rich clinical cohort used in this proposal, with longitudinal, multi-region data for each patient, is ideal for studying the spatio-temporal tumor evolutionary dynamics during treatment. We aim to (1) characterize the genomic, transcriptomic, and proteomic changes associated with HER2- targeted therapy and determine if specific alterations are associated with treatment response, (2) use patient genomic data as input to a spatial computational model of breast tumor evolution under targeted treatment to quantify the patient-specific evolutionary dynamics of resistance, and (3) use insights from the evolutionary model to identify drivers of resistance in a unbiased fashion. This proposal addresses the pressing clinical needs of defining biomarkers to predict which patients will respond to HER2-targeted therapy and understanding how and why treatment resistance develops in others. A deeper understanding of how tumors evolve under targeted therapy using HER2-positive breast cancer as a model system will inform more effective treatment strategies that harness tumor evolution to prevent resistance, with applications to other cancers. The fellowship training will take place at the Stanford University School of Medicine, which has unparalleled research, clinical, and student development resources and emphasizes interdisciplinary research and innovation in both the experimental and computational realms. Dr. Christina Curtis is the ideal sponsor for this proposal due to her significant expertise in tumor evolution, cancer genomics, and biomarker development, her federally funded program in modeling therapeutic resistance in breast cancer, as well as her dedication to student mentorship. Dr. Robert West brings expertise in breast cancer pathology, analysis of archival tissue, and physician-scientist career mentorship. Technical and scientific reasoning abilities will be expanded through research, coursework, and faculty mentoring during the training period, facilitating a successful future career in translational cancer research. Attendance at local and national conferences, together with journal clubs and workshops, will also lead to improved writing, presentation, and networking skills.